Re: PrivateRefCount (for 8.3)

Tom Lane wrote:
> NikhilS <nikkhils(at)gmail(dot)com> writes:
>> What is the opinion of the list as to the best way of measuring if the
>> following implementation is ok?
>> http://archives.postgresql.org/pgsql-hackers/2007-01/msg00752.php
>> As mentioned in earlier mails, this will reduce the per-backend usage of
>> memory by an amount which will be a fraction (single digit percentage)
>> of (NBuffers
>> * int) size. I have done pgbench/dbt2 runs and I do not see any negative
>> impact because of this.
>
> I find it extremely telling that you don't claim to have seen any
> positive impact either.
>
> I think that the original argument
> http://archives.postgresql.org/pgsql-hackers/2006-11/msg00797.php
> is basically bogus. At 500000 buffers (4GB in shared memory) the
> per-backend space for PrivateRefCount is still only 2MB, which is
> simply not as significant as Simon claims; a backend needs at least
> that much for catalog caches etc. There is, furthermore, no evidence
> that running shared_buffers that high is a good idea in the first
> place, or that there aren't other performance bottlenecks that will
> manifest before this one becomes interesting.

hmm - we are continuily running into people with dedicated servers that
have 16GB RAM or even more available and most tuning docs recommend some
20-30% of system RAM to get dedicated to shared_buffers. So having some
500k buffers allocated does not sound so unrealistic in practise and
combined with the fact that people often have a few hundred backends
that could add up to some noticable overhead.
If that is actually a problem given that those people tend to have heaps
of memory is another story but if we can preserve some memory ...

Stefan